Fuel cell system and starting method therefor

ABSTRACT

A fuel cell system and a starting method therefor are capable of setting a start-up mode which is appropriate to energy stored in a secondary battery so as to eliminate problems in starting the fuel cell system. The fuel cell system includes a fuel cell, a secondary battery which is electrically connected with the fuel cell, a secondary-battery charge-amount detection unit which detects an amount of charge in the secondary battery, and a memory which stores at least one threshold value for determining the start-up mode of the fuel cell system. Stored electric energy which corresponds to the amount of charge in the secondary battery is calculated, and a start-up mode of the fuel cell system is determined based on the electric energy stored in the secondary battery and the threshold value stored in the memory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fuel cell systems and driving methodstherefor, and more specifically, to a fuel cell system including asecondary battery which is electrically connected with a fuel cell, anda method of starting the system.

2. Description of the Related Art

Fuel cells take time until they attain a temperature appropriate forpower generation, after being started at an ambient temperature. Whilethe temperature is low, the power generation output of the fuel cell islow. When starting, therefore, fuel cell systems obtain energy frompower supplies other than the fuel cell in order to drive their systemcomponents, etc. Fuel cell systems cannot start themselves without anenergy supply other than the fuel cell. Further, even if a fuel cellsystem has an energy supply such as a secondary battery, a problem willbe encountered during a start-up of the fuel cell system if there is notsufficient supply of energy from the secondary battery until the fuelcell has attained a temperature appropriate for sufficient powergeneration.

A fuel cell system which includes a secondary battery is disclosed inJP-A 9-231991, for example. JP-A 9-231991 discloses a technique forsupplying a load with electric power from a secondary battery when thesystem is started, during which warming-up state of the fuel cell ismonitored. When it is determined that the fuel cell is warmed up to asufficient level, the fuel cell is connected with the load so that thefuel cell supplies electric power to the load.

However, the fuel cell system according to JP-A 9-231991 does notmonitor the amount of energy stored in the secondary battery, andtherefore can fail to start the fuel cell system if sufficient power isnot stored in the secondary battery.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide a fuel cell system and a startingmethod therefor, which are capable of setting a start mode that isappropriate to the energy stored in the secondary battery so as toprevent problems when the system is started.

According to a preferred embodiment of the present invention, a fuelcell system connected with a load includes: a fuel cell; a secondarybattery electrically connected with the fuel cell; a data collectionunit arranged to obtain data concerning electric energy stored in thesecondary battery; and a determination unit arranged to determine one ofa plurality of start-up modes differing from each other in energyconsumption, for the fuel cell system based on the obtained dataconcerning electric energy stored in the secondary battery.

According to another preferred embodiment of the present invention, amethod of starting a fuel cell system connected with a load is provided.The system includes a fuel cell and a secondary battery electricallyconnected with the fuel cell. The method includes the steps of:obtaining data concerning electric energy stored in the secondarybattery; determining one of a plurality of start-up modes differing fromeach other in energy consumption, for the fuel cell system based on theobtained data concerning electric energy stored in the secondarybattery; and operating the fuel cell system in accordance with thedetermined mode.

According to preferred embodiments of the present invention, a start-upmode of a fuel cell system is determined based on data concerningelectric energy stored in the secondary battery, and the fuel cellsystem is operated in accordance with the determined start-up mode. Thismakes it possible to select a start-up mode which is appropriate to theelectric energy stored in the secondary battery, and eliminate problemsin starting the fuel cell system.

Preferably, the start-up mode of the fuel cell system is determinedbased on the data concerning electric energy stored in the secondarybattery and at least one threshold value for determining the start-upmode of the fuel cell system. In this manner, it is possible to selectan optimum start-up mode for the fuel cell system.

Further preferably, the threshold value preferably includes a firstthreshold value representing the energy necessary for starting the fuelcell system. Whether or not to start the fuel cell system is determinedbased on the data concerning electric energy stored in the secondarybattery and the first threshold value. For example, in a case where thedata concerning stored electric energy refers to the stored electricenergy itself, and the first threshold value represents the exact energynecessary for starting the fuel cell system, the fuel cell system isstarted if the electric energy stored in the secondary battery is notsmaller than the first threshold value. On the other hand, if theelectric energy stored in the secondary battery is smaller than thefirst threshold value, starting of the fuel cell system is stopped basedon a judgment that it is impossible to start the fuel cell system. Thismakes it possible to avoid unnecessary consumption of energy.

Further, preferably, the threshold value includes a second thresholdvalue representing normal-consumption energy necessary for starting thefuel cell system in a normal mode. With this arrangement, whether tostart the fuel cell system in the normal mode or in a low consumptionmode is determined based on the data concerning electric energy storedin the secondary battery and the second threshold value. For example, ina case where the data concerning stored electric energy refers to theexact electric energy stored in the secondary battery, and the secondthreshold value represents the normal-consumption energy itself which isnecessary for starting the fuel cell system in the normal mode, the fuelcell system is started in the normal mode if the electric energy storedin the secondary battery is not smaller than the second threshold value.On the other hand, if the electric energy stored in the secondarybattery is smaller than the second threshold value, the fuel cell systemis started in the low consumption mode. In this way, the fuel cellsystem is started in a mode which is appropriate to the electric energystored in the secondary battery.

Preferably, a third threshold value which represents a sum of thenormal-consumption energy necessary for starting the fuel cell system inthe normal mode and a load energy demand necessary for driving the loadnormally is further used. With this arrangement, whether or not to drivethe load normally is determined based on the data concerning electricenergy stored in the secondary battery and the third threshold value.The load is driven normally or in a mode other than normal driving inaccordance with a result of the determination. For example, in a casewhere the data concerning the stored electric energy refers to thestored electric energy itself, and the third threshold value representsthe exact sum of the normal-consumption energy and the load energydemand, the load is enabled for normal driving if the electric energystored in the secondary battery is not lower than the third thresholdvalue. On the other hand, if the electric energy stored in the secondarybattery is smaller than the third threshold value, the load is enabledfor a mode other than normal driving; or more specifically, the load isenabled for a mode where energy consumable by the load is limited, andthe load can be driven under a restrictive condition. As described, theload is enabled for driving within a range allowable by the electricenergy stored in the secondary battery.

Further preferably, when the fuel cell system is started in the lowconsumption mode, a predetermined voltage for determining whether or notthe fuel cell is disconnected from the secondary battery is set to alower value than when started in the normal mode. The determination ofwhether or not the fuel cell is disconnected from the secondary batteryis based on the output voltage of the fuel cell and the predeterminedvoltage. When starting the system in the low consumption mode, thepredetermined voltage is set to a lower value than when started in thenormal mode, whereby the connection between the fuel cell and thesecondary battery is not cut off (the connection is maintained) andcharging of the seconding battery is continued in the low consumptionmode, even in a case where the output voltage of the fuel cell hasreached a value (which is smaller than the predetermined voltage of thenormal mode) at which the connection between the fuel cell and thesecondary battery would be cut off in the normal mode. This makes itpossible to reduce discharge from the secondary battery, i.e., to cutdown on a decrease in the stored electric energy. This arrangementprovides a particular advantage when the fuel cell system is started inan environment where the temperature of the fuel cell is low andtherefore the output voltage of the fuel cell is likely to be low.

Further, preferably, various preferred embodiments of the presentinvention make use of an aqueous solution tank which stores aqueous fuelsolution to be supplied to the fuel cell, an amount of aqueous solutionin the aqueous solution tank is not controlled when the fuel cell systemis started in the low consumption mode. In this case, there is no needfor driving a pump, for example, which makes it possible to reduce powerconsumption.

Preferably, when the fuel cell system is started in the low consumptionmode, the fuel cell is supplied with aqueous fuel solution which has ahigher concentration than a concentration used when the system isstarted in the normal mode, to start power generation. In this case, thetemperature of the fuel cell rises quickly, making possible to shortenthe necessary time to attain the target temperature, although efficiencyis decreased because of increased crossover.

Further preferably, no control is performed for decreasing the amount ofaqueous solution in the aqueous solution tank when the fuel cell systemis started in the low consumption mode. In this case, there is no needto drive a pump, for example, which makes it possible to reduce powerconsumption.

Further, preferably, various preferred embodiments of the presentinvention make use of an air pump which supplies the fuel cell with aircontaining oxygen. When the fuel cell system is started in the lowconsumption mode, the air pump is operated at a lower output than whenstarted in a normal mode, to start power generation. In this case, it ispossible to reduce power consumption by the air pump.

Preferably, various preferred embodiments of the present invention makeuse of an air pump which supplies the fuel cell with air containingoxygen, and an aqueous solution pump which supplies the fuel cell withaqueous fuel solution. When the fuel cell system is started in the lowconsumption mode, flow of air and aqueous fuel solution is reduced, andthe air pump and the aqueous solution pump are driven alternately witheach other. As described, by not driving the air pump and the aqueoussolution pump simultaneously, it becomes possible to reduce powerconsumption by the air pump and the aqueous solution pump, and cut downon a decrease in the electric energy stored in the secondary battery.

Further preferably, when starting the fuel cell system in the lowconsumption mode, the fuel cell system is connected with the load whenthe output voltage of the fuel cell is not lower than a voltage in thesecondary battery. In this case, if the output voltage of the fuel cellbecomes not lower than the voltage of the secondary battery, it ispossible to terminate a no-load operation of the fuel cell even if thefuel cell has not yet attained a predetermined temperature (atemperature serving as a threshold to terminate no-load operation in thenormal mode). This makes it possible to shorten the time for the no-loadoperation and the time to attain the target temperature.

Further, preferably, when the fuel cell system is started in the lowconsumption mode, the fuel cell is operated at a lower output voltagethan when started in a normal mode at the same temperature of the fuelcell. This makes it possible to increase an output current from the fuelcell and to charge the secondary battery quickly.

Preferably, while the load is driven in a mode other than normaldriving, the load is switched to normal driving when electric energystored in the secondary battery is not lower than energy represented bythe third threshold value. This arrangement makes it possible to drivethe load in a mode appropriate to the amount of charge in the secondarybattery.

In fuel cell systems where the fuel cell is supplied with aqueous fuelsolution for power generation, it takes time until the temperature ofaqueous fuel solution rises, i.e. sufficient output is obtained from thefuel cell, since the aqueous fuel solution itself has a large thermalcapacity. Therefore, preferred embodiments of the present invention canbe used suitably in those fuel cell systems where the fuel cell issupplied with aqueous fuel solution for power generation.

Also, preferred embodiments of the present invention can be usedsuitably in transportation equipment which requires that, if the fuelcell system is to be mounted, the capacity of the secondary battery besmall. Specifically, preferred embodiments of the present invention canbe used suitably in cases where at least one of the loads is a motor ofthe transportation equipment.

It should be noted here that the meaning of the term “data concerningstored electric energy” is not limited to the stored electric energyitself, but may mean a value which has a one-to-one relationship withthe stored electric energy (e.g., a value convertible to and from theamount of stored electric energy) such as the amount of electric charge,voltage, current, etc.

The term “energy necessary for starting the fuel cell system” meansenergy necessary for a fuel cell system to start and attain atemperature (target temperature) at which the fuel cell can sufficientlyperform power generation.

The meaning of the term “threshold value corresponding to the energy” isnot limited to energy itself, but may mean a value which has aone-to-one relationship with the energy (e.g., a value convertible toand from the energy) such as the amount of electric charge, voltage,current, etc.

The term “normal mode” is a mode of operating a fuel cell system, whereno restriction is placed on operations of system components, etc., whenthe fuel cell system is started.

The term “low consumption mode” is a mode of operating a fuel cellsystem, where restrictions are placed on operations of systemcomponents, etc., when the fuel cell system is started. Energyconsumption is smaller than in the normal mode.

The term “load energy demand” is energy necessary for driving the loadnormally until a fuel cell attains a temperature (target temperature) atwhich the fuel cell can sufficiently perform power generation.

“Normal drive (or to drive normally)” means driving without restriction.

The above-described and other elements, features, steps,characteristics, aspects and advantages of the present invention willbecome clearer from the following detailed description of preferredembodiments to be made with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a primary portion of a fuel cellsystem according to a preferred embodiment of the present invention.

FIG. 2 is a perspective view showing a state where the fuel cell systemis mounted on a frame of a motorbike.

FIG. 3 is a schematic diagram showing a primary portion of the fuel cellsystem.

FIG. 4 is a block diagram showing an electric configuration of the fuelcell system.

FIG. 5( a) is a graph showing time course changes of a fuel celltemperature after a fuel cell system is started; FIG. 5( b) is a graphshowing a relationship between a temperature of a fuel cell when a fuelcell system is started and a necessary time to attain a targettemperature.

FIG. 6 is a circuit diagram showing a voltage control unit.

FIG. 7 is a flowchart showing an example of main operation performedwhen the fuel cell system is started.

FIG. 8 is a graph for describing a case where the fuel cell system isstarted in a low consumption mode but a vehicle is not enabled fordriving.

FIG. 9 is a graph for describing a case where the fuel cell system isstarted normally and the vehicle is enabled for restrictive driving.

FIG. 10 is a graph for describing a case where the fuel cell system isstarted normally and the vehicle is enabled for normal driving.

FIGS. 11( a) and 11(b) include graphs showing cases where a restrictionis made on vehicle's output: FIG. 11( a) shows a case where maximummotor current is limited; FIG. 11( b) shows a case where maximum motoroutput is limited.

FIG. 12 is a graph showing that stored electric energy necessary in asecondary battery varies depending upon the temperature at a time whenthe fuel cell system is started.

FIG. 13 is a flowchart showing an operation in a case where the fuelcell system is started in a low consumption mode but the vehicle is notenabled for driving.

FIG. 14 is a flowchart showing an operation in a case where the fuelcell system is started in a normal mode and the vehicle is enabled forrestrictive driving.

FIG. 15 is a flowchart showing an operation in a case where the fuelcell system is started in the normal mode and the vehicle is enabled fornormal driving.

FIG. 16 is a flowchart showing an operation performed at a time whenpower generation is started.

FIG. 17 is a flowchart showing a process of determining an alarm level.

FIG. 18 is a flowchart showing a process of controlling an amount ofaqueous solution in an aqueous solution tank.

FIG. 19 is a flowchart showing a process of controlling a concentrationof aqueous methanol solution.

FIG. 20 is a flowchart showing a process of decreasing the amount ofaqueous solution in the aqueous solution tank.

FIG. 21 is a flowchart showing a process of controlling an aqueoussolution pump and an air pump.

FIG. 22 is a flowchart showing a process of controlling an outputvoltage of a fuel cell.

FIG. 23( a) is a graph showing a temperature of a fuel cell and outputvoltage with respect to operating time in a normal mode; FIG. 23( b) isa graph showing the temperature of the fuel cell and output voltage withrespect to the operating time in a low consumption mode.

FIG. 24 is a schematic diagram for describing another preferredembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereinafter, preferred embodiments of the present invention will bedescribed with reference to the drawings.

As shown in FIG. 1 through FIG. 4, a fuel cell system 10 according to apreferred embodiment of the present invention is provided as a directmethanol fuel cell system. Since direct methanol fuel cell systems donot need a reformer, the systems are used suitably for equipment whichrequires portability, and equipment in which size reduction isdesirable. Herein, description will be made for a case where the fuelcell system 10 is used in a motorbike taken as an example oftransportation equipment. It should be noted here that the motorbikewill be shown only as a motorbike frame 200 as in FIG. 2. In FIG. 2, theleft side is the front side of the vehicle and the right side is therear side thereof. The fuel cell system 10 is disposed along themotorbike frame 200. Hereinafter, the motorbike may be referred to as avehicle as necessary.

Referring mainly to FIG. 1, the fuel cell system 10 includes a fuel cell12. The fuel cell 12 is configured as a fuel cell stack constituted by aplurality of fuel cells connected (layered) in series. Each fuel cellincludes an electrolyte 12 a provided by a solid polymer film, as wellas an anode (fuel electrode) 12 b and a cathode (air electrode) 12 cwhich sandwich the electrolyte 12 a.

Also, the fuel cell system 10 includes a fuel tank 14 which storeshighly concentrated methanol fuel (aqueous methanol solution containingmethanol at approximately 50 wt %) F. The fuel tank 14 is connected withan aqueous solution tank 18 which stores aqueous methanol solution S,via a fuel supply pipe 16. The fuel supply pipe 16 is provided with afuel pump 20, and as the fuel pump 20 is driven, methanol fuel F in thefuel tank 14 is supplied to the aqueous solution tank 18.

The fuel tank 14 is provided with a level sensor 15 arranged to detect aheight of liquid surface of the methanol fuel F in the fuel tank 14.Likewise, the aqueous solution tank 18 is provided with a level sensor22 arranged to detect a height of liquid surface of the aqueous methanolsolution S in the aqueous solution tank 18. By detecting the height ofliquid surfaces with the level sensors 15, 22, the amount of liquids inthe tanks can be detected. The same applies to a level sensor 54 to bedescribed later.

The aqueous solution tank 18 is connected with the anode 12 b of thefuel cell 12 via an aqueous solution pipe 24. The aqueous solution pipe24 is provided with an aqueous solution pump 26, a radiator 28 whichfunctions as a heat exchanger, and an aqueous solution filter 30, inthis order from the upstream side. A cooling fan 32 is disposed near theradiator 28 in order to cool the radiator 28. An aqueous methanolsolution S in the aqueous solution tank 18 is pumped toward the anode 12b by the aqueous solution pump 26, cooled by the radiator 28 asnecessary, and further, purified by the aqueous solution filter 30 andthen supplied to the anode 12 b.

On the other hand, the cathode 12 c in the fuel cell 12 is connectedwith an air pump 34 via an air-side pipe 36. The air-side pipe 36 isprovided with an air filter 38. Therefore, air from the air pump 34which contains oxygen (oxidizer) is supplied to the cathode 12 c afterit is purified by the air filter 38.

Also, the anode 12 b and the aqueous solution tank 18 are connected witheach other via a pipe 40, through which the aqueous solution tank 18receives unused aqueous methanol solution and produced carbon dioxidedischarged from the anode 12 b.

Further, the cathode 12 c is connected with a water tank 44 via a pipe42. The pipe 42 is provided with a radiator 46 which functions as agas-liquid separator, and near the radiator 46 a cooling fan 48 forcooling the radiator 46 is disposed. The cathode 12 c discharges anexhaust which contains moisture (water and water vapor). The exhaust issupplied to the water tank 44 via the pipe 42.

Also, the aqueous solution tank 18 and the water tank 44 are connectedwith each other via a CO₂ vent pipe 50. The CO₂ vent pipe 50 is providedwith a methanol trap 52 for separating aqueous methanol solution S. Withthis arrangement, carbon dioxide discharged from the aqueous solutiontank 18 is supplied to the water tank 44.

The water tank 44 is provided with a level sensor 54 that is arranged todetect a height of liquid in the water tank 44. Also, an exhaust gaspipe 56 is attached to the water tank 44. Through the exhaust gas pipe56, carbon dioxide and exhaust from the cathode 12 c are discharged.

The water tank 44 is connected with the aqueous solution tank 18 via awater returning pipe 58. The water returning pipe 58 is provided with awater pump 60. Water in the water tank 44 is returned to the aqueoussolution tank 18 by driving the water pump 60, as necessary, dependingon the situation in the aqueous solution tank 18.

Also, along the aqueous solution pipe 24, a bypass pipe 62 is providedbetween the radiator 28 and the aqueous solution filter 30.

Referring also to FIG. 4, further, in the fuel cell system 10, thebypass pipe 62 is provided with a concentration sensor 64 arranged todetect the concentration of aqueous methanol solution S, and an aqueoussolution temperature sensor 65 arranged to detect the temperature ofaqueous methanol solution S. The fuel cell 12 is provided with a celltemperature sensor 66 that is arranged to detect the temperature of fuelcell 12, and an ambient temperature sensor 68 that is arranged to detectthe ambient temperature is provided near the air pump 34. It should benoted here that the cell temperature sensor 66 is disposed at a positionin the fuel cell 12 where the highest temperature will be observed, e.g.near an outlet of the aqueous methanol solution S.

As shown in FIG. 4, the fuel cell system 10 includes a control circuit70.

The control circuit 70 preferably includes: a CPU 72 for performingnecessary calculations and providing control over operations of the fuelcell system 10; a clock circuit 74 which provides the CPU 72 with clocksignals; a volatile memory 76 including, e.g., a DRAM for keepingelapsed time based on the clock signals provided to the CPU 72, flags,calculation data, etc; a non-volatile memory 78 including, e.g., anEPROM or an SRAM for storing programs, data, etc, to control operationsof the fuel cell system 10; a reset IC 80 for preventing erroneousoperation of the fuel cell system 10; a plurality of interface circuits82 a through 82 r for connection with external components; a voltagedetection circuit 84 for detecting an output voltage of the fuel cell12; a current detection circuit 86 for detecting an output electriccurrent of the fuel cell 12; a voltage adjustment circuit 88 foradjusting the output voltage of the fuel cell 12; a voltage protectioncircuit 92 for protecting the electric circuit 90 from over-voltage; adiode 94 placed in the electric circuit 90 for protecting the fuel cell12; a power source circuit 96 for providing the electric circuit 90 witha voltage for a normal mode; and a power source circuit 98 for providingthe electric circuit 90 with a voltage for a low consumption mode. Thefuel cell system 10 is preferably configured as a series type systemwhich supplies a load with electric power via a secondary battery 108(to be described later).

The voltage detection circuit 84, the current detection circuit 86, thevoltage adjustment circuit 88 and the diode 94 constitute a voltagecontrol unit 100.

The CPU 72 of the control circuit 70 as described above is supplied withdetection signals from the concentration sensor 64, the aqueous solutiontemperature sensor 65, the cell temperature sensor 66 and the ambienttemperature sensor 68, via the interface circuits 82 a, 82 b, 82 c and82 d, respectively. In addition, the CPU 72 is supplied with detectionsignals from the level sensors 15, 22 and 54 via the interface circuits82 l, 82 k and 82 o respectively. Further, the CPU 72 is supplied withdetection signals from a rollover switch 102 which detects rollover, viathe interface circuit 82 n, as well as signals from an input section 104which is used for making various settings and information entry, via theinterface circuit 82 p.

Also, the CPU 72 sends control signals to the fuel pump 20, the aqueoussolution pump 26, the air pump 34, the heat-exchanger cooling fan 32,the gas-liquid separator cooling fan 48 and the water pump 60 via theinterface circuits 82 j, 82 g, 82 h, 82 f, 82 e and 82 i, respectively.Hence the CPU 72 controls these system components. Also, the CPU 72sends control signals to a display unit 106 via the interface circuit 82q, and controls the display unit 106 which is a unit for displaying avariety of information and providing the rider of the motorbike with avariety of information.

Also, the fuel cell 12 is connected with the secondary battery 108placed in a battery box 107. The secondary battery 108 complements theoutput from the fuel cell 12, is charged with electric energy from thefuel cell 12, and discharges the electric energy to supply power to amotor 116 (to be described later) and system components. Particularly,when starting power generation, system components are driven by electricenergy from the secondary battery 108, and as the amount of powergenerated by the fuel cell 12 increases, the electric energy is storedin the secondary battery 108. The secondary battery 108 is preferably anickel hydride battery, a lithium ion battery, a Ni—Cd battery, etc. Thesecondary battery 108 is connected with a control device 110. Thecontrol device 110 is constituted by a CPU, a memory, etc., and includesa secondary-battery charge-amount detection unit 112 which detects anamount of charge in the secondary battery 108, and can also detect avoltage, current, temperature, etc. of the secondary battery 108. In thepresent embodiment, the amount of charge in the secondary battery 108 isobtained by multiplying the voltage of the secondary battery with apredetermined constant; however, the calculation may also includefactors which reflect consideration into the electric current and theextent of battery deterioration. The control device 110 sends thesepieces of information about the secondary battery 108 to the controlcircuit 70 via an interface circuit 113, and also to a motor controller114 connected with the secondary battery 108. The motor controller 114is connected to a load, i.e. a motor 116 of the motorbike, and theelectric energy supplied to the motor 116 is controlled by the motorcontroller 114. The motor controller 114 is connected with a meter 118for measuring various data of the motor 116. Information such as datameasured by the meter 118, state of the motor 116, etc. is inputted tothe CPU 72 via the interface circuit 113 of the control device 110 andthe interface circuit 82 m of the control circuit 70.

In this preferred embodiment, the volatile memory 76 stores such dataas: the amount of charge in the secondary battery 108; electric energystored in the secondary battery 108; detected temperatures of the fuelcell 12; a necessary time for the fuel cell 12 to attain a targettemperature; a first threshold value used to determine whether or notthe fuel cell system 10 should be started; a second threshold value usedto determine whether the fuel cell system 10 should be started in normalmode or in low consumption mode; a third threshold value used todetermine whether or not the load may be driven normally; a load energydemand; etc.

The non-volatile memory 78 stores data such as: low-consumption electricpower necessary for driving the fuel cell system 10 in low consumptionmode for a unit of time; normal electric power necessary for driving thefuel cell system 10 in normal mode for a unit of time, a predeterminedvoltage for determining whether or not the fuel cell 12 should bebrought to a no-load state; a vehicle average output which represents aspecific unit of energy necessary for driving the load normally for aunit of time, etc. The non-volatile memory 78 also stores table datawhich indicates a relationship between the temperature of the fuel cell12 when starting the fuel cell system 10 and a necessary time forattaining a target temperature (for example, approximately 65° C. in thepresent preferred embodiment). The necessary time is calculated on thebasis of the temperature of the fuel cell 12 at the time of start-up,power generation efficiency and thermal capacity. For example, as shownin FIGS. 5( a) and 5(b), the higher the temperature of the fuel cell 12at the time of system start-up, the shorter will be the necessary timeto the target temperature. Once the temperature of the fuel cell 12 atthe time of start-up is detected, a necessary time for the fuel cell 12to attain the target temperature is estimated by making reference to thetable data. In FIG. 5( a), fuel cell temperatures at the time point zero(on the vertical axis) each indicate the temperature at the time ofstart-up.

The non-volatile memory 78 also stores control information (controlparameters, programs, etc.) for a plurality of start-up modes ofdifferent energy consumption.

In the present preferred embodiment, the CPU 72 defines the firstdetermination unit and the second determination unit, whereas thevolatile memory 76 and the non-volatile memory 78 define the memory.

Reference will be made here to FIG. 6, to describe the voltage controlunit 100.

The current detection circuit 86, provided by a current transformer forexample, detects an output current from the fuel cell 12. The currentdetected by the current detection circuit 86 is converted into a voltageand is supplied to the CPU 72. The current detection circuit 86 isconnected, on its output side, with a voltage detection circuit 84 whichdetects an output voltage of the fuel cell 12. The detected outputvoltage of the fuel cell 12 is supplied to the CPU 72. The voltagedetection circuit 84 also detects a voltage of the secondary battery108. Further, the voltage detection circuit 84 is provided, on itsoutput side, with a voltage adjustment circuit 88 which includes an FET1and an FET2. The CPU 72 supplies control signals to the gate in each ofthe FET1 and the FET2, and the output voltage of the fuel cell 12 isadjusted on the basis of the control signals. Further, the voltageadjustment circuit 88 is connected, on its output side, with the diode94 for protecting the fuel cell 12.

The voltage control unit 100 which has the configuration as described isconnected, on its output side, with the secondary-battery charge-amountdetection unit 112. The secondary-battery charge-amount detection unit112 detects the amount of charge in the secondary battery 108.

A power generation operation by the fuel cell system 10 as the abovewill be outlined. As an unillustrated main switch is turned on, the fuelcell system 10 drives its system components such as the aqueous solutionpump 26 and the air pump 34, thereby starting the power generation(operation).

When starting the power generation, the aqueous solution pump 26 isdriven, whereby the aqueous methanol solution S of a desiredconcentration stored in the aqueous solution tank 18 is pumped towardthe fuel cell 12, cooled by the radiator 28 as necessary, purified bythe aqueous solution filter 30, and then supplied to the anode 12 b. Onthe other hand, air which contains oxygen serving as an oxidizer ispumped by the air pump 34 toward the fuel cell 12, purified by the airfilter 38, and then supplied to the cathode 12 c.

At the anode 12 b in the fuel cell 12, methanol and water in the aqueousmethanol solution S react electrochemically with each other to producecarbon dioxide and hydrogen ions. The produced hydrogen ions flowthrough the electrolyte 12 a, to the cathode 12 c. The hydrogen ionsreact electrochemically with oxygen in the air supplied to the cathode12 c, to produce water (water vapor) and electric energy.

Carbon dioxide produced at the anode 12 b in the fuel cell 12 flowsthrough the pipe 40, the aqueous solution tank 18 and the CO₂ vent pipe50, to the water tank 44, and then discharged from the exhaust gas pipe56.

Meanwhile, most of the water vapor produced on the cathode 12 c in thefuel cell 12 is liquefied and discharged in the form of water, withsaturated water vapor being discharged in the form of gas. Part of thewater vapor which was discharged from the cathode 12 c is cooled andliquefied as its temperature decreases to or below the dew point in theradiator 46. Liquefaction of the water vapor by the radiator 46 isaccelerated by operation of the cooling fan 48. Water (liquid water andwater vapor) from the cathode 12 c and unused air are supplied to thewater tank 44 via the pipe 42. Also, water which moved to the cathode 12c due to water crossover is discharged from the cathode 12 c, andsupplied to the water tank 44. Further, water and carbon dioxide whichare produced at the cathode 12 c from methanol crossover are dischargedfrom the cathode 12 c, and supplied to the water tank 44.

It should be noted here that water crossover is a phenomenon in which afew mols of water moves to the cathode 12 c, accompanying the hydrogenions which are produced at the anode 12 b and are moving to the cathode12 c. Methanol crossover is a phenomenon in which methanol moves to thecathode 12 c, accompanying the hydrogen ions which move to the cathode12 c. At the cathode 12 c, methanol reacts with air supplied from theair pump 34, and thereby decomposes into water and carbon dioxide.

Water (liquid) which was collected in the water tank 44 is returned asappropriately by the water pump 60, through the water returning pipe 58,to the aqueous solution tank 18, where the water is used for themethanol aqueous solution S.

Next, with reference to FIG. 7, description will be made for an exampleof main operation in the fuel cell system 10 at a time of start up. Inthe present preferred embodiment, the fuel cell system 10 has threestart-up modes, a normal mode, a low consumption mode and no start-up.Energy consumed in each mode is different.

First, as an unillustrated main switch is turned ON, an amount of charge(remaining capacity) in the secondary battery 108 is detected and storedin the volatile memory 76 (Step S1). The secondary-battery charge-amountdetection unit 112 in the control device 110 detects a voltage of thesecondary battery, and by multiplying the secondary-battery voltage witha predetermined constant, the amount of charge in the secondary battery108 is obtained. The CPU 72 multiplies the obtained amount of charge inthe secondary battery 108 with a predetermined voltage, to calculateelectric energy stored in the secondary battery 108 (amount ofcharge×voltage=stored electric energy) (Step S2), and the value isstored in the volatile memory 76. In the present preferred embodiment,the data gathering unit arranged to obtain the stored electric energydata includes the secondary-battery charge-amount detection unit 112 andthe CPU 72. It should be noted here that the amount of charge in thesecondary battery 108 may be obtained on the basis of the voltage of thesecondary battery detected by the voltage detection circuit 84.

Then, the cell temperature sensor 66 detects a temperature of the fuelcell 12 (Step S3). It should be noted here that the temperature of thefuel cell 12 refers to a temperature which corresponds to the output ofthe fuel cell 12; and as a substitution for the temperature of the fuelcell 12, the system may use the temperature of aqueous methanol solutionS in the aqueous solution tank 18 which has a high thermal capacity, orthe temperature of exhaust from the cathode 12 c, etc.

Next, reference is made to the table data which is stored in thenon-volatile memory 78. The table data indicates a relationship betweenthe temperature of the fuel cell 12 at a time of start-up and necessarytime to attain a target temperature. Based on the detected temperatureof the fuel cell 12, a necessary amount of time for attaining the targettemperature is estimated (Step S5).

The estimated necessary time is multiplied by power consumption which isrequired for driving the fuel cell system 10 in low consumption mode fora unit of time (low-consumption electric power), whereby low-consumptionenergy which is used as a first threshold value is calculated (necessarytime×low-consumption electric power=low-consumption energy) (Step S7).In the present preferred embodiment, the low-consumption electric poweris approximately 70 W, for example, and most of it is consumed by theair pump 34 and a headlight of the vehicle.

Then, the system checks on whether or not the electric energy stored inthe secondary battery 108 is smaller than the low-consumption energy(the first threshold value) (Step S9). If the electric energy stored inthe secondary battery 108 is smaller than the low-consumption energy,the system determines that the start-up is impossible, stops startingthe fuel cell system 10, and disables the vehicle (Step S11).

On the other hand, if Step S9 determines that the electric energy storedin the secondary battery 108 is not smaller than the low-consumptionenergy, the system determines that the start-up is possible, andcalculates normal-consumption energy which is used as a second thresholdvalue (Step S13). The normal-consumption energy is calculated bymultiplying the estimated necessary time by normal electric power whichis required for driving the fuel cell system 10 in normal mode for aunit of time (necessary time×normal electric power=normal-consumptionenergy).

Then, the system checks on whether or not the electric energy stored inthe secondary battery 108 is smaller than the normal-consumption energy(the second threshold value) (Step S15). If the electric energy storedin the secondary battery 108 is smaller than the normal-consumptionenergy, the system determines that it is impossible to start in thenormal mode. Thus, the system starts the fuel cell system 10 in the lowconsumption mode. However, driving of the vehicle is disabled (StepS17). As described, the fuel cell system 10 can be started even if theelectric energy stored in the secondary battery 108 is not very large.

On the other hand, if Step S15 determines that the electric energystored in the secondary battery 108 is not smaller than thenormal-consumption energy, the system determines that a start-up in thenormal mode is possible, and the process goes to Step S19.

In Step S19, the vehicle's average output (about 800 W, for example) innormal operation, which is represented by the unit amount of energystored in the non-volatile memory 78, is multiplied by the necessarytime to the target temperature, to obtain a load energy demand (vehicleaverage output×necessary time=load energy demand). The load energydemand and the normal-consumption energy are added to each other, to beused as a third threshold value (Step S21).

Then, the system checks on whether or not the electric energy stored inthe secondary battery 108 is smaller than the sum of the load energydemand and the normal-consumption energy (the third threshold value)(Step S23). If the electric energy stored in the secondary battery 108is smaller, restriction will be placed on vehicle driving, and thesystem sets the amount of the restriction (Step S25). In the presentpreferred embodiment, the restriction is placed on driving of thevehicle rear wheel. The restriction may be made in steps for example,with the amounts of restriction predetermined, and the setting beingmade to an amount appropriate to the electric energy stored in thesecondary battery 108. Then, the fuel cell system 10 is started in thenormal mode, and the vehicle is enabled for driving under a restrictivecondition (Step S27).

On the other hand, if Step S23 determines that the electric energystored in the secondary battery 108 is not smaller than the sum of theload energy demand and the normal-consumption energy, the fuel cellsystem 10 is started in the normal mode, and the vehicle is enabled fornormal driving, so that the vehicle can be driven normally (Step S29).

It should be noted here that in the operation described above, thestart-up mode of the fuel cell system 10 and vehicle driving status maybe displayed in the display unit 106.

Now, reference will be made to FIGS. 8( a) and 8(b) to describe a casewhere the fuel cell system 10 is started in the low consumption mode butthe vehicle is disabled.

If the electric energy stored in the secondary battery 108 has aninitial value as shown in FIG. 8( a), starting the fuel cell system 10in the normal mode will result in a time course as indicated by inBroken Line A1. More specifically, the electric energy stored in thesecondary battery 108 will be zero during the start-up, becoming unableto continue the start-up process of fuel cell system 10 even if thevehicle is not driven. In this case therefore, the fuel cell system 10is started not in the normal mode but in the low consumption mode wherepower generation is started with special limitations placed on the powerconsumed by the system components. Then, the electric energy stored inthe secondary battery 108 will be as shown in Solid Line B1. It shouldbe noted here that the limitations on the power consumed by the systemcomponents will be implemented by reducing the necessary time to attainthe target temperature and limiting operation of the system components,for example.

In FIG. 8( b), Broken Line A2 shows the output of the fuel cell 12 whenthe system is started in normal mode, Solid Line B2 shows the output ofthe fuel cell 12 when the system is started in low consumption mode,Broken Line A3 shows the power consumption by the fuel cell system 10when the system is started in normal mode, Solid Line B3 shows the powerconsumption by the fuel cell system 10 when the system is started in thelow consumption mode, and Solid Line B4 shows vehicle's average outputwhen it is not driven.

As will be understood from Broken Line A2 and Solid Line B2, the lowconsumption mode will require a long time before the output of the fuelcell 12 reaches the normal level. Also, with reference to Broken Line A3and Solid Line B3, the start-up in normal mode will require a powerconsumption of approximately 150 W, for example, by the systemcomponents whereas the start-up in low consumption mode will decreasethe power consumption by the system components to approximately 100 W,for example, making it possible to reduce energy consumption.

Next, reference will be made to FIGS. 9( a) and 9(b) to describe a casewhere the fuel cell system 10 is started in the normal mode and thevehicle is enabled for restrictive driving.

If the electric energy stored in the secondary battery 108 has aninitial value as shown in FIG. 9( a), starting the fuel cell system 10in the normal mode will result in a time course as indicated by BrokenLine C1. More specifically, since the electric energy stored in thesecondary battery 108 is small, the electric energy stored in thesecondary battery 108 will be zero during the start-up, and it willbecome unable to continue the start-up process of fuel cell system 10.In this case therefore, the fuel cell system 10 is started in the normalmode but vehicle driving is restricted. Then, the electric energy storedin the secondary battery 108 will be as shown in Solid Line D1.

In FIG. 9( b), Solid Line D2 shows the output of the fuel cell 12 whenthe system is started in normal mode, Solid Line D3 shows the powerconsumption by the fuel cell system 10 when the system is started innormal mode, Solid Line D4 shows a vehicle's average output when thevehicle is driven under a restricted condition, and Broken Line C4 showsthe vehicle's average output when it is driven normally. In thisexample, the vehicle's average output is restricted from a stateindicated by Broken Line C4 to a state indicated by Solid Line D4.

Now, reference will be made to FIGS. 10( a) and 10(b) to describe a casewhere the fuel cell system 10 is started in normal mode and the vehicleis enabled for normal driving.

If the electric energy stored in the secondary battery 108 has aninitial value as shown in FIG. 10( a), the fuel cell system 10 isstarted in normal mode and the vehicle is enabled for normal driving.Then, as shown in Solid Line E1, the electric energy stored in thesecondary battery 108 will decrease due to energy consumption by thesystem components of fuel cell system 10 and by the vehicle until acertain time point t is reached. After the time point t, the output fromthe fuel cell 12 stabilizes at a level not lower than the amount ofenergy consumed by the system components and the vehicle, where there isno longer energy deficit from the secondary battery 108. Thus, thesystem components and the vehicle are driven by the output from the fuelcell 12, and the secondary battery 108 starts to be charged. In thiscase, the secondary battery 12 has a surplus in its stored electricenergy, and it is possible to start the system in the normal mode and todrive the vehicle normally.

In FIG. 10( b), Solid Line E2 shows the output of the fuel cell 12 whenthe system is started in normal mode, Solid Line E3 shows the powerconsumption by the fuel cell system 10 when the system is started innormal mode, and Solid Line E4 shows a vehicle's average output when thevehicle is driven normally.

It should be noted here that the vehicle's output will fluctuate inactual situations since the vehicle will be moving or stopping atdifferent times. FIG. 8( b), FIG. 9( b) and FIG. 10( b) show vehicle'saverage outputs.

Next, FIGS. 11( a) and 11(b) show an example where a restriction isplaced on the vehicle's output.

FIG. 11( a) shows an example where the vehicle's output is restricted bylimiting a maximum current of the motor 116. From FIG. 11( a), it isclear that limiting a maximum current of the motor 116 reduces drivepower of the rear wheel, making it possible to reduce energyconsumption.

FIG. 11( b) shows an example where the vehicle's output is restricted bylimiting a maximum output of the motor 116. It is clear that limiting amaximum output of the motor 116 reduces drive power of the rear wheel asindicated by hatching in FIG. 11( b), making it possible to reduceenergy consumption.

Also, as understood from FIG. 12, the amount of electric energy whichmust be stored in the secondary battery 12 depends upon the temperatureat the time of starting the fuel cell 12. Specifically, if thetemperature at the time of start-up is 20° C., stored electric energy F1is necessary. At 30° C., stored electric energy F2 is necessary, and at40° C., stored electric energy F3 is necessary. In essence, the amountof stored electric energy required in the secondary battery 108 issmaller if the temperature at the time of start-up is higher. It shouldbe noted here that the amounts of stored electric energy F1 through F3each represent stored electric energy which is necessary for startingthe fuel cell system 10 in the normal mode, with the vehicle enabled fornormal driving.

Next, with reference to FIG. 13, description will be made for asubroutine in Step S17, FIG. 7, i.e. an operation in the case where thefuel cell system 10 is started in low consumption mode but the vehicleis disabled.

The fuel cell system 10 is started in the low consumption mode, powergeneration is started (Step S51), and the process then moves to normaloperation (Step S53).

On the other hand, the vehicle, i.e. the load, is disabled at first(Step S55). Specifically, no voltage is applied to the motor 116, thusthe motor 116 is not drivable, and this state is maintained until StepS57 determines that the electric energy stored in the secondary battery108 is not lower than the normal-consumption energy (the secondthreshold value). In other words, only a charging operation of thesecondary battery 108 is performed until the secondary battery 108 hasbeen charged to a certain extent. Once the electric energy stored in thesecondary battery 108 is not lower than the normal-consumption energy,the vehicle is enabled for driving under a restricted condition (forexample, with a limit on a maximum current of the motor 116) (Step S59).This condition for driving the vehicle is maintained until Step S61determines that a new calculation of the electric energy stored in thesecondary battery 108 is not lower than the sum (the third thresholdvalue) of the normal-consumption energy and the load energy demand. Whenthe electric energy stored in the secondary battery 108 is not lowerthan the sum of the normal-consumption energy and the load energydemand, the restriction is removed, and the vehicle is enabled fornormal driving (Step S63).

As described, once the electric energy stored in the secondary battery108 is not lower than the third threshold value, the load is switched tonormal driving. With this arrangement, it is possible to drive the loadin a mode appropriate to the electric energy stored in the secondarybattery 108.

Next, with reference to FIG. 14, description will be made for asubroutine in Step S27, FIG. 7, i.e. an operation in the case where thefuel cell system 10 is started in the normal mode and the vehicle isenabled for restrictive driving.

As for the fuel cell system 10, first, the level sensor 54 detects theamount of liquid (amount of water) in the water tank 44 (Step S101). Ifthe amount of liquid detected in Step S101 is not smaller than a firstpredetermined amount (250 cc for example) which is a value set inadvance (Step S103: YES), the water pump 60 is driven by the power fromthe secondary battery 108, to return water from the water tank 54through the water returning pipe 58, to the aqueous solution tank 18(Step S105). Thereafter, when the amount of liquid detected by the levelsensor 54 is not greater than a second predetermined amount (220 cc forexample) which is a value set in advance (Step S107: YES), the waterpump 60 is stopped (Step S109).

Also, even if the amount of liquid detected by the level sensor 54 isgreater than the second predetermined amount in Step S107 (Step S107:NO), the process goes to Step S109 after a lapse of a predeterminedamount of time (Step S111: YES). As described, the water pump 60 isstopped after a lapse of a predetermined amount of time, eliminating aproblem that the second predetermined amount is never detected and powergeneration is never started due to a malfunction in the level sensor 54,for example. The operation in Step S105 is continued until thepredetermined amount of time has lapsed (Step S111: NO).

After Step S109, system components such as the fuel pump 20, the aqueoussolution pump 26, the air pump 34, the heat-exchanger cooling fan 32,the gas-liquid separator cooling fan 48 and the water pump 60 aredriven, and power generation in the normal mode is started (Step S113).If Step S103 determines that the amount of liquid in the water tank 44is smaller than the first predetermined amount (Step S103: NO), theprocess goes to Step S113. As described, normal operation is allowed(Step S115) after power generation in the normal mode is started.

On the other hand, the vehicle, i.e. the load, is enabled forrestrictive driving (for example, with a limit on a maximum current ofthe motor 116) at first (Step S117). This condition for driving thevehicle is maintained until Step S119 determines that a new calculationof the electric energy stored in the secondary battery 108 gives a valuenot lower than the sum (the third threshold value) of thenormal-consumption energy and the load energy demand. When the electricenergy stored in the secondary battery 108 is not lower than the sum ofthe normal-consumption energy and the load energy demand, the vehicle isenabled for normal driving (Step S121).

Further, reference will be made to FIG. 15 to describe a subroutine inStep S29, FIG. 7, i.e. an operation in the case where the fuel cellsystem 10 is started in normal mode and the vehicle is enabled fornormal driving.

As for the fuel cell system 10, first, the level sensor 54 detects theamount of liquid (amount of water) in the water tank 44 (Step S151). Ifthe amount of liquid detected in Step S151 is not smaller than the firstpredetermined amount (250 cc for example) which is a value set inadvance (Step S153: YES), the water pump 60 is driven by the power fromthe secondary battery 108, to return water from the water tank 54through the water returning pipe 58 into the aqueous solution tank 18(Step S155). Thereafter, when the amount of liquid detected by the levelsensor 54 is not greater than the second predetermined amount (220 ccfor example) which is a value set in advance (Step S157: YES), the waterpump 60 is stopped (Step S159).

Also, even if the amount of liquid detected by the level sensor 54 isgreater than the second predetermined amount in Step S157 (Step S157:NO), the process goes to Step S159 after a lapse of a predeterminedamount of time (about a minute, for example) (Step S161: YES). Asdescribed, the water pump 60 is stopped after a lapse of a predeterminedamount of time, eliminating a problem that the second predeterminedamount is never detected and power generation is never started due to amalfunction in the level sensor 54, for example. The operation in StepS155 is continued until the predetermined amount of time has lapsed(Step S161: NO).

After Step S159, system components such as the fuel pump 20, the aqueoussolution pump 26, the air pump 34, the heat-exchanger cooling fan 32,the gas-liquid separator cooling fan 48 and the water pump 60 aredriven, and power generation in the normal mode is started (Step S163).If Step S153 determines that the amount of liquid in the water tank 44is smaller than the first predetermined amount (Step S153: NO), theprocess goes to Step S163. As described, normal operation is allowed(Step S165) after power generation in the normal mode is started.

On the other hand, no limitation is set to the output of the vehicle,i.e. the load, and normal driving is allowed from the first place (StepS167).

Further, with reference to FIG. 16, description will cover the operationin Step S51 of FIG. 13, Step S113 of FIG. 14 and Step S163 of FIG. 15performed at the time power generation is started.

First, the system is brought to a state of no load (Step S201).Specifically, the voltage adjustment circuit 88 opens the electriccircuit 90 to drive the fuel cell 12 with no load, and the connectionbetween the fuel cell 12 and the secondary battery 108 is cut off. Underthis state, tapping of electric current from the fuel cell 12 isstopped. Then, an alarm level is determined (Step S202). Thereafter, theamount of aqueous solution in the aqueous solution tank 18 is controlled(Step S203), the concentration of aqueous methanol solution S iscontrolled (Step S205), and the amount of aqueous solution in theaqueous solution tank 18 is decreased (Step S207). Further, the aqueoussolution pump 26 and the air pump 34 are controlled (Step S209), and theoutput voltage of the fuel cell 12 is controlled (Step S211).

The operation in Steps S201 through S211 in FIG. 16 will be described inmore specifically.

Reference will be made to FIG. 17, to describe a process in Step 202 inFIG. 16, of determining an alarm level.

First, mode detection is performed to see if the current mode is thenormal mode or the low consumption mode (Step S251), and based on thedetected mode, a predetermined voltage (a lowest voltage at whichoperation can be performed without damaging the cell) is selected (StepS253). The predetermined voltage as a conversion into a single cellvoltage (a voltage in one fuel cell) would be about 0.25V for normalmode and about 0.2V for low consumption mode, for example.

As described, when starting the system in low consumption mode, a lowervalue is set for the predetermined voltage than when starting the systemin normal mode, whereby the connection between the fuel cell 12 and thesecondary battery 108 is not cut off (the connection is maintained) andcharging to the secondary battery 108 is continued in the lowconsumption mode, even in cases where the output voltage of the fuelcell 12 reaches a value at which the connection between the fuel cell 12and the secondary battery 108 would be cut off in the normal mode. Thismakes it possible to reduce discharge from the secondary battery 108,i.e. to cut down on a decrease in the stored electric energy.

Reference will now be made to FIG. 18 to describe the process in StepS203 in FIG. 16, of controlling the amount of aqueous solution in theaqueous solution tank 18.

First, the mode is checked (Step S301). In normal mode, the systemchecks on whether or not the amount of aqueous solution detected in theaqueous solution tank 18 by the level sensor 22 is smaller than apredetermined amount of the aqueous solution tank (the amount of aqueoussolution in the aqueous solution tank 18 during power generation, whichmay be about one liter, for example) (Step S303). If smaller, the levelsensor 54 detects the amount of liquid (amount of water) in the watertank 44 (Step S305), and the system checks on whether or not thedetected amount of liquid is not smaller than a first predeterminedamount (250 cc for example) (Step S307). If the detected amount ofliquid is not smaller than the first predetermined amount, the waterpump 60 is driven and water is returned to the aqueous solution tank 18(Step S309). This operation is continued until Step S311 determines thata predetermined amount of time has passed, and the process goes back toStep S303 if the predetermined amount of time has passed.

If Step S303 determines that the amount of aqueous solution in theaqueous solution tank 18 is not smaller than the predetermined amount,or Step S307 determines that the amount of liquid is smaller than thefirst predetermined amount, the water pump 60 is stopped (Step S313).

On the other hand, if the detected mode is the low consumption mode, nocontrol of the amount of aqueous solution in the aqueous solution tank18 is performed.

As described, there is no need for driving the water pump 60 whenstarting in low consumption mode, since no control of the amount ofaqueous solution in the aqueous solution tank 18 is done. Thus, it ispossible to reduce power consumption.

Referring to FIG. 19, description will now cover the concentrationcontrol on aqueous methanol solution S in Step S205 in FIG. 16. In thisprocess, the concentration of the aqueous methanol solution S ispreferably set to be higher than the concentration for the normaloperation.

First, mode detection is made to see if the current mode is normal modeor low consumption mode (Step S351). Then, the concentration sensor 64detects the concentration of aqueous methanol solution S (Step S353),and the system checks on whether or not the detected concentration ofaqueous methanol solution S is lower than a predetermined concentrationfor the detected mode (Step S355). The predetermined concentration isassigned to each mode, and the concentration value is different betweenthe normal mode and the low consumption mode. Although the predeterminedconcentration in the normal mode varies depending upon the temperatureof the fuel cell 12, an ambient temperature, etc., the value is higherthan the concentration for the normal operation. On the other hand, inlow consumption mode, the value is even higher than the setting for thenormal mode by about 2 wt % to about 5 wt %. As an example, when theambient temperature is about 20° C., the predetermined concentration isset to about 6% for normal mode and about 8% for low consumption mode.

If Step S355 determines that the concentration of aqueous methanolsolution S is lower than the predetermined concentration, the fuel pump20 is driven (Step S357). The operation is continued until Step S359determines that a predetermined amount of time has passed. When thepredetermined amount of time has passed, the process goes back to StepS353. If Step S355 determines that the concentration of aqueous methanolsolution S is not lower than the predetermined concentration, the fuelpump 20 is stopped (Step S361).

As described, when starting the system in low consumption mode, powergeneration is started with a supply of aqueous methanol solution S tothe fuel cell 12 at a higher concentration than when starting the systemin normal mode. Although this increases crossover and decreasesefficiency, temperature rises quickly, making it possible to shorten thenecessary time to attain the target temperature.

Referring to FIG. 20, description will now cover the process ofdecreasing the amount of aqueous solution in the aqueous solution tank18 in Step S207 in FIG. 16.

First, the mode is checked (Step S401). In normal mode, the water pump60 is driven to move aqueous methanol solution S from the aqueoussolution tank 18 to the water tank 44, whereby the amount of aqueousmethanol solution S in the aqueous solution tank 18 is decreased (StepS403). On the other hand, the water pump 60 is not driven in lowconsumption mode, i.e., the control process of decreasing the amount ofaqueous methanol solution S in the aqueous solution tank 18 is notperformed.

As described, there is no need for driving the water pump 60 whenstarting the system in low consumption mode because the control processof decreasing the amount of aqueous methanol solution in the aqueoussolution tank 18 is not performed. This makes possible to decrease powerconsumption.

Referring to FIG. 21, description will now cover the process ofcontrolling the aqueous solution pump 26 and the air pump 34 in StepS209 in FIG. 16.

First, mode detection is made to see if the current mode is normal modeor low consumption mode (Step S451). Then, the system determines anamount of air flow to be supplied by the air pump 34 for the detectedmode (Step S453). For example, in a normal mode, the amount of air flowto be supplied by the air pump 34 is set to approximately three timesthe theoretical demand, and approximately two times the theoreticaldemand in a low consumption mode 1. It should be noted here that theamount of air flow to be supplied by the air pump 34 in the lowconsumption mode 1 is preferably not smaller than about 20% and smallerthan about 100% of the value for the normal mode. Next, the systemdetermines an amount of flow of aqueous methanol solution S to besupplied by the aqueous solution pump 26, for the detected mode (StepS455). For example, in the normal mode the amount of flow of aqueoussolution to be supplied by the aqueous solution pump 26 is set to thesame amount as in normal power generation, whereas the flow is set to aminimum required in the low consumption mode 1.

Then, in the normal mode or the low consumption mode 1, the air pump 34is driven, and the amount of air flow determined for the particular modeis supplied to the cathode 12 c of the fuel cell 12 (Step S457).Likewise, the aqueous solution pump 26 is driven, and the amount of flowof aqueous methanol solution S determined for the mode is supplied tothe anode 12 b of the fuel cell 12 (Step S459).

In a low consumption mode 2, the aqueous solution pump 26 and the airpump 34 are driven alternately with each other (Step S461). Thisprevents excessive voltage drop caused by driving both of the pumpssimultaneously.

As described, power consumption by the air pump 34 can be decreased whenstarting in the low consumption mode 1 by starting power generation witha lower output of the air pump 34 than in the normal mode.

When starting in the low consumption mode 2, the amount of flow isdecreased in the supply of air and aqueous methanol solution S, and theair pump 34 and the aqueous solution pump 26 are driven alternately andnot simultaneously. This makes it possible to reduce power consumptionby the air pump 34 and the aqueous solution pump 26, and therefore tocut down on a decrease in the electric energy stored in the secondarybattery 108.

Referring to FIG. 22, description will now cover the process ofcontrolling the output voltage of the fuel cell.

First, mode detection is made to see if the current mode is normal modeor low consumption mode (Step S501).

In low consumption mode, the system checks on whether or not the outputvoltage of the fuel cell 12 is not lower than the voltage of thesecondary battery 108 (Step S503), and the process waits until theoutput voltage of the fuel cell 12 is not lower than the voltage of thesecondary battery 108. When the output voltage of the fuel cell 12 isnot lower than the voltage of the secondary battery 108, the outputvoltage of the fuel cell 12 is set to V1 which is a value for the lowconsumption mode (Step S505).

In normal mode on the other hand, the system checks on whether or notthe temperature of the fuel cell 12 has reached a predeterminedtemperature (Step S507), and the process waits until the temperature ofthe fuel cell 12 has reached the predetermined temperature. When thetemperature of the fuel cell 12 has reached the predeterminedtemperature, the process goes to Step S505, and the output voltage ofthe fuel cell 12 is set to V1 which is the value for the normalconsumption mode. The output voltage of the fuel cell 12 is set by thevoltage adjustment circuit 88.

Then, a temperature T of the fuel cell 12 is checked (Step S509), andthe output voltage of the fuel cell 12 is set, based on the mode and thetemperature T. If the temperature T is not higher than T1, the processwaits until a predetermined amount of time has passed (Step S511). Whenthe predetermined amount of time has passed, the system checks onwhether or not the output voltage of the fuel cell 12 is smaller than apredetermined voltage (Step S513). In terms of single-cell voltage, thesystem checks, for example, if the single-cell voltage is lower thanabout 0.25V for the normal mode, or if the single-cell voltage is lowerthan about 0.2V for the low consumption mode. If the output voltage ofthe fuel cell 12 is lower than the predetermined voltage, the processgoes back to Step S201 in FIG. 16, where the system is set to no load,and tapping of the electric current from the fuel cell 12 is stopped. Onthe other hand, if the output voltage of the fuel cell 12 is not lowerthan the predetermined voltage, tapping of electric current from thefuel cell 12 is continued, and the process goes back to Step S509.

If Step S509 determines that the temperature T is higher than T1 and nothigher than T2, the output voltage of the fuel cell 12 is set to V2(Step S515), and the process goes to Step S511. When the temperature Tis higher than T2, the output voltage of the fuel cell 12 is set to V3(Step S517), and the system checks on whether or not the temperature Tof the fuel cell 12 has reached the target temperature (normal operationtemperature) (Step S519). If the temperature T has not yet reached thetarget temperature, the process goes to Step S511, whereas if the targetis reached, the process returns and brings the fuel cell system 10 tonormal operation. The predetermined temperatures in the presentpreferred embodiment are approximately: T1=50° C., T2=60° C., and thetarget temperature=65° C. Also, the single-cell voltages correspondingto the voltages V1, V2 and V3 are approximately 0.50V, 0.40V and 0.35Vfor normal mode, respectively, while being approximately 0.40V, 0.35Vand 0.25V for low consumption mode, respectively. Lowering the outputvoltage of the fuel cell 12 makes it possible to increase the chargecurrent to the secondary battery 108.

As described, when starting in low consumption mode, no-load operationof the fuel cell 12 is terminated and the output voltage of the fuelcell 12 is set to V1 once the output voltage of the fuel cell 12 is notlower than the voltage of the secondary battery 108 even if the fuelcell 12 has not yet attained a predetermined temperature. Thisarrangement makes it possible to shorten the time of no-load operationand the time to attain the target temperature.

FIG. 23( a) shows the temperature of the fuel cell 12 and the outputvoltage of the fuel cell 12 in the normal mode. FIG. 23( b) shows thetemperature of the fuel cell 12 and the output voltage of the fuel cell12 in the low consumption mode.

From FIGS. 23( a) and 23(b), it is understood that switching fromno-load operation to an operation at the output voltage V1 from the fuelcell 12 takes place at an earlier time point in low consumption modethan in normal mode. This is because, as described above, the systemwill set the output voltage of the fuel cell 12 to V1 in the lowconsumption mode as soon as the output voltage of the fuel cell 12 isnot lower than the voltage of the secondary battery 108. In the normalmode, the system is still in no-load operation at this point.

Also, when starting the system in low consumption mode, output voltagesetting values V1, V2 and V3 from the fuel cell 12 are lower than thosewhen starting the system in normal mode at the same fuel celltemperature. This makes it possible to increase the output current fromthe fuel cell 12 in low consumption mode over the output current innormal mode, and thereby to charge the secondary battery 108 quickly.The temperature increase in the fuel cell 12 is quicker, too, and it ispossible to switch to normal operation at an earlier time.

According to the fuel cell system 10 as described, a start-up mode ofthe fuel cell system 10 is determined on the basis of electric energystored in the secondary battery 108 and a threshold value obtained fromcalculation, and the fuel cell system 10 is operated in accordance withthe determined start-up mode. This makes it possible to select anoptimum start-up mode suitable for the electric energy (amount ofcharge) stored in the secondary battery 108, eliminating problems whenstarting the fuel cell system 10.

Specifically, the amount of charge in the secondary battery 108 isconverted into an amount of stored electric energy, and this storedelectric energy is compared to the first threshold value which is thelow-consumption energy itself, i.e., the amount of energy necessary forstarting the fuel cell system 10 in low consumption mode. If theelectric energy stored in the secondary battery 108 is not smaller thanthe first threshold value, the fuel cell system 10 is started. On theother hand, if the electric energy stored in the secondary battery 108is smaller than the first threshold value, the system determines thatthe fuel cell system 10 cannot be started even in the low consumptionmode, and stops starting the fuel cell system 10. This makes it possibleto avoid unnecessary energy consumption.

Also, if the electric energy stored in the secondary battery 108 is notsmaller than the second threshold value which is the normal-consumptionenergy itself, i.e. the amount of energy which is necessary to start thefuel cell system 10 in normal mode, the fuel cell system 10 is startedin normal mode. On the other hand, if the electric energy stored in thesecondary battery 108 is smaller than the second threshold value, thefuel cell system 10 is started in low consumption mode. Following theprocess described above, it is possible to start the fuel cell system 10in a mode appropriate for the electric energy stored in the secondarybattery 108.

Further, if the electric energy stored in the secondary battery 108 isnot smaller than the third threshold value which is the very sum of thenormal-consumption energy and the load energy demand, the vehicle isenabled for normal driving. On the other hand, if the electric energystored in the secondary battery 108 is smaller than the third thresholdvalue, the vehicle is enabled for driving in a mode other than thenormal driving. As described, the vehicle is made drivable within arange allowable by the electric energy stored in the secondary battery108.

The fuel cell system 10 described above is suitably used in vehicleswhich require that the capacity of the secondary battery 108 be small.

It should be noted here that in the above-described preferredembodiments, the thresholds are preferably provided by values of energy,and the threshold values are preferably compared to the electric energystored in the secondary battery 108. However, the present invention isnot limited to this. The thresholds may be provided by the amount ofcharge, so that those threshold values are compared to the amount ofcharge in the secondary battery 108. In this case, energy is convertedto the amount of charge so that it can be used as a threshold value.Also, the thresholds may be provided by the voltage, so that thosethreshold values are compared to the voltage values of the secondarybattery 108; or the thresholds may be provided by the current, so thatthose threshold values are compared to the values of current flowing inthe secondary battery 108.

In the preferred embodiments described above, threshold values fordetermining the start-up mode are preferably obtained by calculation.However, this may be replaced by the following arrangement. For example,as shown in FIG. 24, three threshold values A, B and C are predeterminedfor the amount of charge in the secondary battery 108, to define fourcategories. In this case, after the main switch is turned ON, the amountof charge in the secondary battery 108 is detected, the category inwhich the amount of charge falls is found, and a process assigned tothis particular category is performed.

Specifically, if the amount of charge is not greater than the thresholdvalue A, the fuel cell system 10 is not started, or the vehicle is notenabled, either. If the amount of charge is greater than the thresholdvalue A and not greater than the threshold value B, the fuel cell system10 is started in low consumption mode but the vehicle is not enabled. Ifthe amount of charge is greater than the threshold value B and notgreater than the threshold value C, the fuel cell system 10 is startedin normal mode and the vehicle is enabled for restrictive driving. Ifthe amount of charge is greater than the threshold value C, the fuelcell system 10 is started in the normal mode and the vehicle is enabledfor normal driving.

According to the various preferred embodiments of the present invention,the start-up mode can be set easily.

It should be noted here that operation speed of the CPU 72 may belowered in low consumption mode to reduce power consumption.

In the preferred embodiments described above, it is preferable thatthree threshold values are used and four operation modes are defined forthe fuel cell system 10 and the vehicle. However, the present inventionis not limited to this.

For example, three operation modes may be defined by using a thresholdvalue D which represents a normal amount of energy necessary for makinga normal start of the fuel cell system 10, and a threshold value E whichrepresents a sum of the normal amount of energy necessary for startingof the fuel cell system 10 in normal mode and the load energy demandnecessary for making normal driving of the load (D<E). In this case, forexample, if the amount of charge in the fuel cell 12 is not greater thanthe threshold value D, the fuel cell system 10 is not started, or thevehicle is not enabled, either. If the amount of charge is greater thanthe threshold value D and not greater than the threshold value E, thefuel cell system 10 is started in normal mode and the vehicle is enabledfor restrictive driving. If the amount of charge is greater than thethreshold value E, the fuel cell system 10 is started in normal mode andthe vehicle is enabled for normal driving. It should be noted here thatthe threshold values D, E may be obtained by calculation based on thetemperature of fuel cell 12 or may be predetermined.

Further, the start-up mode of the fuel cell system 10 may be determinedon the basis of data concerning electric energy stored in the secondarybattery 108, without using threshold values.

Preferred embodiments of the present invention are used suitably indirect methanol fuel cell systems in which it takes time for raising atemperature of aqueous fuel solution, i.e. until a sufficient output isobtained from the fuel cell.

In the preferred embodiments described above, methanol is preferablyused as fuel and aqueous methanol solution is preferably used as fuelaqueous solution. However, the present invention is not limited by this,and the fuel may be provided by other alcoholic fuels such as ethanol,and the aqueous fuel solution may be provided by aqueous solutions ofthe alcohol, such as aqueous ethanol solution.

In the preferred embodiments described above, description is made for acase where a motorbike is used as a load. However, the present inventionis not limited to this. The load may be provided by any transportationequipment other than motorbikes, such as automotive vehicles includingfour-wheeled automobiles, marine vessels and aircraft.

The present invention is applicable also to fuel cell systems mountedwith a reformer, and fuel cell systems where hydrogen is supplied to thefuel cell. Further, the present invention is applicable to small,stationary-type fuel cell systems.

The present invention being thus far described and illustrated indetail, it should be noted that the description and drawings onlyrepresent examples of preferred embodiments of the present invention,and should not be interpreted as limiting the present invention. Thespirit and scope of the present invention is only limited by words usedin the accompanied claims.

1. A fuel cell system connected with a load, the fuel cell systemcomprising: a fuel cell; a secondary battery electrically connected withthe fuel cell; a data gathering unit to obtain data concerning electricenergy stored in the secondary battery; a determination unit todetermine one of a plurality of start-up modes, differing from eachother in energy consumption, for the fuel cell system based on theobtained data concerning electric energy stored in the secondarybattery; and a memory programmed to store at least one threshold valueused to determine the one of the plurality of start-up modes of the fuelcell system, wherein the determination unit determines the one of theplurality of start-up modes of the fuel cell system based on the dataconcerning electric energy stored in the secondary battery and the atleast one threshold value stored in the memory; wherein the at least onethreshold value includes a first energy start threshold valuerepresenting energy necessary to start the fuel cell system, and thedetermination unit includes a start determination unit to determinewhether or not to start the fuel cell system based on the dataconcerning electric energy stored in the secondary battery and the firstenergy start threshold value; and the at least one threshold valueincludes a second energy start threshold value representingnormal-consumption energy necessary to start the fuel cell system in anormal mode, and the start determination unit determines whether tostart the fuel cell system in the normal mode or in a low consumptionmode based on the data concerning electric energy stored in thesecondary battery and the second energy start threshold value.
 2. Thefuel cell system according to claim 1, wherein the memory is programmedto store a third energy start threshold value representing a sum of thenormal-consumption energy necessary to start the fuel cell system in thenormal mode and a load energy demand necessary to drive the loadnormally, and the fuel cell system further comprises anotherdetermination unit to determine whether or not to drive the loadnormally based on the data concerning electric energy stored in thesecondary battery and the third energy start threshold value.
 3. Thefuel cell system according to claim 1, wherein the fuel cell isconfigured to receive aqueous fuel solution and use the aqueous fuelsolution to generate power.
 4. The fuel cell system according to claim1, wherein the load includes at least a motor of transportationequipment.
 5. A method of starting a fuel cell system connected with aload, the system including a fuel cell and a secondary batteryelectrically connected with the fuel cell, the method comprising thesteps of: obtaining data concerning electric energy stored in thesecondary battery; determining one of a plurality of start-up modesdiffering from each other in energy consumption for the fuel cell systembased on the obtained data concerning electric energy stored in thesecondary battery; and operating the fuel cell system in accordance withthe determined mode; wherein the step of determining one of a pluralityof start-up modes includes the steps of: determining the one of theplurality of start-up modes of the fuel cell system based on the dataconcerning electric energy stored in the secondary battery and at leastone threshold value; and determining whether or not to start the fuelcell system based on the data concerning electric energy stored in thesecondary battery and a first energy start threshold value representingenergy necessary to start the fuel cell system, wherein the at least onethreshold value includes the first energy start threshold value; and apredetermined voltage for determining whether or not the fuel cell isdisconnected from the secondary battery is set to a lower value whenstarting the fuel cell system in a low consumption mode than whenstarting in a normal mode, the determination on whether or not the fuelcell is disconnected from the secondary battery being made based on anoutput voltage of the fuel cell and the predetermined voltage.
 6. Themethod of starting a fuel cell system according to claim 5, wherein theat least one threshold value includes a second energy start thresholdvalue representing normal-consumption energy necessary to start the fuelcell system in the normal mode, and whether to start the fuel cellsystem in the normal mode or in the low consumption mode beingdetermined based on the data concerning electric energy stored in thesecondary battery and the second energy start threshold value.
 7. Themethod of starting a fuel cell system according to claim 5, furthercomprising the step of using a second energy start threshold valuerepresenting a sum of normal-consumption energy necessary for startingthe fuel cell system in the normal mode and a load energy demandnecessary for driving the load normally, and whether or not to drive theload normally being determined based on the data concerning electricenergy stored in the secondary battery and the second energy startthreshold value, the load being driven normally or in a mode other thannormal driving in accordance with a result of the determination.
 8. Themethod of starting a fuel cell system according to claim 7, wherein theload being driven in a mode other than normal driving is switched tonormal driving when electric energy stored in the secondary battery isnot lower than energy represented by the second energy start thresholdvalue.
 9. The method of starting a fuel cell system according to claim5, further comprising the step of using an aqueous solution tank forstoring aqueous fuel solution to be supplied to the fuel cell, whereinan amount of aqueous solution in the aqueous solution tank is notcontrolled when the fuel cell system is started in the low consumptionmode.
 10. The method of starting a fuel cell system according to claim5, wherein the fuel cell is supplied with aqueous fuel solution having ahigher concentration when the fuel cell system is started in the lowconsumption mode than a concentration used when the system is started inthe normal mode, to start power generation.
 11. The method of starting afuel cell system according to claim 5, further comprising the step ofusing an aqueous solution tank for storing aqueous fuel solution to besupplied to the fuel cell, wherein no control is performed fordecreasing an amount of aqueous solution in the aqueous solution tankwhen the fuel cell system is started in the low consumption mode. 12.The method of starting a fuel cell system according to claim 5, furtherusing an air pump for supplying the fuel cell with air containingoxygen, wherein the air pump is operated at a lower output when the fuelcell system is started in the low consumption mode than when started inthe normal mode, to start power generation.
 13. The method of starting afuel cell system according to claim 5, further comprising the step ofusing an air pump for supplying the fuel cell with air containingoxygen, and an aqueous solution pump for supplying the fuel cell withaqueous fuel solution, wherein the air pump and the aqueous solutionpump are driven alternately with each other when the fuel cell system isstarted in the low consumption mode.
 14. A method of starting a fuelcell system connected with a load, the system including a fuel cell anda secondary battery electrically connected with the fuel cell, themethod comprising the steps of: obtaining data concerning electricenergy stored in the secondary battery; determining one of a pluralityof start-up modes differing from each other in energy consumption forthe fuel cell system based on the obtained data concerning electricenergy stored in the secondary battery; and operating the fuel cellsystem in accordance with the determined mode; wherein the step ofdetermining one of a plurality of start-up modes includes the steps of:determining the one of the plurality of start-up modes of the fuel cellsystem based on the data concerning electric energy stored in thesecondary battery and at least one threshold value; and determiningwhether or not to start the fuel cell system based on the dataconcerning electric energy stored in the secondary battery and a firstenergy start threshold value representing energy necessary to start thefuel cell system, wherein the at least one threshold value includes thefirst energy start threshold value; and the fuel cell system isconnected with the load when an output voltage of the fuel cell is notlower than a voltage in the secondary battery when starting the fuelcell system in a low consumption mode.
 15. A method of starting a fuelcell system connected with a load, the system including a fuel cell anda secondary battery electrically connected with the fuel cell, themethod comprising the steps of: obtaining data concerning electricenergy stored in the secondary battery; determining one of a pluralityof start-up modes differing from each other in energy consumption forthe fuel cell system based on the obtained data concerning electricenergy stored in the secondary battery; and operating the fuel cellsystem in accordance with the determined mode; wherein the step ofdetermining one of a plurality of start-up modes includes the steps of:determining the one of the plurality of start-up modes of the fuel cellsystem based on the data concerning electric energy stored in thesecondary battery and at least one threshold value; and determiningwhether or not to start the fuel cell system based on the dataconcerning electric energy stored in the secondary battery and a firstenergy start threshold value representing energy necessary to start thefuel cell system, wherein the at least one threshold value includes thefirst energy start threshold value; and the fuel cell is operated at alower output voltage when the fuel cell system is started in a lowconsumption mode than when started in a normal mode at a sametemperature of the fuel cell.